There is known a fixture for positioning two parts (cf. laid-open Application of the Federal Republic of Germany No. 2,537,146; F 16 B 5/02) wherein precision taper holes containing locating members in the form of centering balls are provided on the side of mating faces.
The known fixture ensures stable positioning of both parts with respect to each other and a rigid joint between them, which results due to an interference set up between said ball and the walls of taper holes by means of fasteners.
However, the load sustained by every locating member of the known fixture is concentrated at a point or along a line of a length equalling the width of the area of contact between the ball and the walls of a taper hole. As a result, the rigidity of the joints is low.
Moreover, taper holes in parts with very hard mating faces must be finished on jig grinders capable of machining only one hole at a time. This prevents several operations being performed on a workpiece at a given time and consequently increases labour requirements for part manufacturing.
When balls are used as locating members, no joint making various angles with the horizontal can be assembled without recourse to additional contrivances, and in the process of disassembly on the horizontal plane it is difficult to remove the ball from the taper hole for the ball centre is located in this case contiguously with a mating face of a datum member. The cleaning of taper holes from dust, chips and coolant is a problem as well.
Also known are assorted datum and locating members of versatile fixtures with a variable setup (cf. USSR Inventor's Certificate No. 795,861; B23 Q3/00), a salient feature of which is datum members each provided with a hole for a fastener and with at least two intersecting grooves which are located in a side with mating faces and form, after the mating faces have been joined, unified spaces wherein which one contained locating members in the form of prismatic keys. The grooves have a stepped outline and the pitch of the grooves equals their width. The prismatic keys are also of a stepped configuration, one step being off-set relative to the base of another step, and are held fast to the datum members by screws.
An assembling of the known fixture is a time-consuming job, for every key is held fast by a separate screw. The known fixture is also labour consuming in manufacture, for tapped holes must be made in the datum members to receive the screws holding down the keys. Unavoidable clearances exist between the keys and the walls of the grooves, to which the datum members can be displaced, depriving the workpiece of firm positioning.